Ion implantation has become the technology preferred by industry to dope semiconductors with impurities in the large-scale manufacture of integrated circuits. High-energy ion implanters are used for deep implants into a substrate. Such deep implants are required to create, for example, retrograde wells. Eaton GSD/HE and GSD/VHE ion implanters are examples of such high-energy implanters. These implanters can provide ion beams at energy levels up to 5 MeV (million electron volts). U.S. Pat. No. 4,667,111, assigned to the assignee of the present invention, Eaton Corporation, describes such a high-energy ion implanter.
Ion implanters operate at very high voltage levels. Typically, ions in the beam are accelerated and decelerated by electrodes and other components in the implanter that reside at differing voltage levels. For example, positive ions are extracted from an ion source and accelerated by electrodes having increasingly negative potentials. In a high-energy ion implanter, the ion beam accelerates as it passes through a radio frequency (RF) linear accelerator (linac). The ion beam progresses through the RF linac by passing through a series of acceleration stages (resonator modules) in which accelerating fields are produced by synchronizing the frequency of the RF voltage to the ion beam velocity.
In known RF linac resonator modules, an RF signal is coupled to a low-voltage end of an inductor coil, and an accelerating electrode is directly coupled to an opposing high-voltage end of the inductor coil. Each accelerating electrode is mounted between two grounded electrodes and separated by gaps. When the resonator module achieves a state of resonance, a sinusoidal voltage of large magnitude is provided at the location of the accelerating electrode.
The accelerating electrode and the ground electrodes on either side operate in a known "push-pull" manner to accelerate the ion beam passing therethrough, which has been "bunched" into "packets". During the negative half cycle of the RF sinusoidal electrode voltage, a positively charged ion packet is accelerated (pulled by the accelerating electrode from the upstream grounded electrode across the first gap). At the transition point in the sinusoidal cycle, wherein the electrode voltage is neutral, the packet drifts through the electrode (also referred to as a "drift tube") and is not accelerated. During the positive half cycle of the RF sinusoidal electrode voltage, positively charged ion packets are further accelerated (pushed by the accelerating electrode) toward the downstream grounded electrode across the second gap. This push-pull acceleration mechanism is repeated at subsequent resonator modules having accelerating electrodes that also oscillate at a high-voltage radio frequency, thereby further accelerating the ion beam packets by adding energy thereto.
In each of the first and second accelerating gaps, electric field lines produce radial focusing in the first gap and radial defocusing in the second gap. If the gap is operating at a phase which keeps the particles bunched in the axial direction, more often than not the electric field is increasing in magnitude, through its RF cycle, when a particle is passing through the gap. Consequently, the electric radial defocusing forces in the second gap are greater than the radial focusing forces in the first gap, resulting in a net radial defocusing as the ion beam passes through a particular resonator module.
Accordingly, to refocus the ion beam, magnetic or electrostatic quadruples are 20 positioned intermediate each of resonator modules in an RF linac. These magnetic or electrostatic quadruples include a plurality of magnets or high-voltage electrodes, respectively, through or by which the ion beam passes. In the case of an electrostatic quadrupole, the high-voltage electrodes, which operate between +20 kilovolts (KV) and -20 KV, are typically made of graphite, which is subject to sputtering when struck by the ion beam. Sputtered graphite material tends to coat the insulating standoffs (e.g., alumina (Al.sub.2 O.sub.3)) that mount the high-voltage electrode to the electrically grounded quadrupole housing. If a sufficient amount of sputtered material coats the standoffs, it can lead to voltage breakdown between the electrode and the grounded housing by creating an electrical current path therebetween.
It is an object of the present invention, then, to provide ion implanter components that reduce the chance of being coated with material that can cause electrical shorts and resulting arcing. It is another object of the invention to provide such components in the form of electrical insulators that mount electrodes within the beamline. It is a further object to provide such components in the form of insulating standoffs that are used to mount electrostatic quadrupole electrodes in the linear accelerator portion of a high-energy ion implanter.